The present invention relates to a programming current controlling method and device for an electrically erasable programmable semiconductor memory cell. The invention is directed, more particularly, to a programming current controlling device which is formed on a chip containing a plurality of such memory cells in an array.
The programming current for an EPROM depends both on circuit elements coupled to the EPROM cell as well as on the characteristics of the EPROM cell itself. Since the programming load line normally chosen passes through a steeply sloped portion of the curve and is itself steeply sloped any relatively small variation in the curve results in a relatively significant variation in programming current. The programming current must be maintained between an I.sub.MIN value determined by the minimum current required to program the EPROM cell and I.sub.MAX set by the I.sub.pp specification. In the case of low programming current long programming times result whereas high programming currents may fail the specification for maximum allowable programming current.
Advances in EPROM bit density have resulted in a trend towards shorter programming pulses per byte to minimize end user programming time per device. Because the programming time is directly proportional to the programming current there is a conflicting requirement between the need for faster programming time and the requirement to limit the maximum source/drain current to keep it below the specification maximum.
It is therefore an object of the present invention to provide an improved method and apparatus for controlling the programming current in an array of EPROMS. It is a further object of the present invention to provide an EPROM memory device the programming of which is substantially independent of the variations in its fabrication process.